This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 09 162.5, filed on Mar. 3, 1999, the entire disclosure of which is incorporated herein by reference.
The invention relates to a method for reducing vibrations transmitted by a wheel unit to a vehicle to which the wheel unit is mounted. The invention further relates to an apparatus for reducing such vibrations.
The wheel units of a vehicle are guided by guide arms and support the sprung mass of the vehicle by means of spring or damping devices. The term xe2x80x9cwheel unitxe2x80x9d as used here refers to a tire mounted on a wheel rim. Irregularities in a tire, such as localized variations in the tire elasticity, dimensional deviations, unbalances and other irregularities in the wheel rim and/or tire characteristics of the wheel unit can cause undesirable vibrations that are transmitted to other components of the vehicle. Vibrations that are transmitted to the steering wheel of a vehicle are particularly noticeable and disturbing.
Various efforts have been made to reduce these vibrations to a tolerable level or to eliminate them completely. Today, the wheel unit is generally dynamically balanced on conventional balancing machines or is balanced after it has been assembled on a vehicle.
Wheel unit balancing can be supplemented in that, preceding a balancing operation, geometrical characteristics and/or material properties of the wheel rim and/or the tire unit are optimized before dynamically balancing that wheel unit. Such optimization takes into consideration that the wheel rim, the tire and the wheel unit may show radial and transverse run-outs and that the spring characteristics of the tire can vary locally.
U.S. Pat. No. 4,244,416 (Newton) describes a tire buffing machine and a method for applying a load to a tire that is mounted on a wheel rim to determine a radial run-out. Rubber is then ground from places on the tire that have been found to show a run-out due to excess rubber. The wheel unit is then balanced on a dynamic balancing machine.
German Patent DE 41 43 543 C2 teaches a method for reducing operational vibration characteristics of wheel units. A wheel unit is mounted on a wheel balancing machine, the radial run-out of the wheel rim is measured and determined under a constant load applied to the wheel unit while rotating at approximately 60 r.p.m. The constant load is applied by pressing a loading drum against the tire of the wheel unit. It is then determined whether rotating the tire around the periphery of the wheel rim will bring improvement or whether it is necessary to grind the periphery of the tire for obtaining improvements. The appropriate measures are then carried out and the wheel unit is then balanced.
Both of these methods include the use of several devices, which are often a source of inaccuracies. The grinding process is time-consuming, costly and environmentally undesirable because rubber dust is produced. Besides, the prior art leaves room for improvement, especially with regard to simulating actual vehicle characteristics or vehicle operating conditions.
In view of the above it is the aim of the invention to achieve the following objects singly or in combination:
to provide a method for reducing vibrations that are caused by a wheel unit;
to provide a method that takes optimally into account actual vehicle operating conditions by simulating these conditions on a combined balancing and loading machine;
to allocate different degrees of influence to various parameters that together constitute the actual vehicle operating conditions for determining the required balancing weights;
to provide an apparatus for applying to a wheel unit load conditions which closely resemble the actual vehicle operating conditions;
to provide an apparatus that will provide balance information for a wheel unit with regard to both transverse and radial non-uniformities or unbalances in the wheel unit geometry or in the material of the wheel unit; and
to combine a balancing machine with a wheel loading machine so that the balancing information can be obtained under simulated operating conditions.
The above objects have been achieved by the method according to the invention which is characterized by the following steps:
a) mounting a wheel unit including a tire on a wheel rim to a wheel mounting of a measuring unit for rotation;
b) rotating said wheel unit about a wheel unit axis (W) at an r.p.m. within an actual vehicle operation r.p.m. range;
c) simulating actual vehicle operating conditions by loading said rotating wheel unit through a loading device forming a shock absorber having a spring damping characteristic corresponding substantially to a spring damping characteristic of an actual vehicle wheel suspension;
d) measuring vibration values and respective angular values while said wheel unit is rotating under said loading; and
e) processing said vibration values and said angular values for providing unbalance information for a compensating operation.
The present method is performed by an apparatus according to the invention comprising:
a) a balancing machine 1 including a wheel mounting for said vehicle wheel unit 3, said wheel mounting having a rotation axis W,
b) a drive for rotating said vehicle wheel unit,
c) a loading device 10 in the form of a shock absorber for applying a load to said vehicle wheel unit, said shock absorber of said loading service comprising spring and damping elements 14, 15 for applying the load against a tread of said tire, said spring and damping elements 14, 15 having a spring damping characteristic corresponding substantially to a spring damping characteristic of an actual vehicle wheel suspension for simulating actual vehicle operating conditions, and
d) sensors 6, 7 for measuring vibration values and respective angular values to obtain compensating information.
For the first time, according to the invention, practically all factors that significantly influence the vibrational behavior of a wheel unit during vehicle operation are taken into account for reducing vibrations in a single balancing set-up in a single apparatus. The invention is based on the recognition that a number of parameters must be taken into account for a proper balancing operation. The parameters include for example: the influence of the wheel suspension on vibrations, the translational inertia masses, the rotational inertial masses, and the deformation of the tire at its tread contact area with the road. All these parameters exert a significant influence on the quality of the reduction of the vibration that can be achieved. On the one hand, depending on the circumstances of the individual case, such as a particular type of wheel or the wheel suspension in the vehicle, varying degrees of significance can be assigned to these parameters for achieving the desired vibration reduction. On the other hand the same significance or weight may be accorded to the parameters. For example, variations in the stiffness of the tire can cause vibrations of magnitudes that correspond to the vibrations caused by wheel unbalances determined in a load-free dynamic balancing. By including, for example, the inertia mass and rotational inertia mass of the vehicle wheel unit in the measurements for the balancing, the variations in stiffness and the influence of the mass moment of inertia of the vehicle wheel unit are also taken into consideration. Both of these parameters are causes for vibrations and are superposed on each other. Furthermore, wheel conditions under which vibrations should not be noticeable are taken into account by including the inertia mass of the wheel unit and of the unsprung wheel assembly components such as the hub, brake, and the wheel carrier or mounting, the spring rate and the damping of the wheel suspension, in the balancing measurements. Such measurements are preferably taken at a wheel speed that corresponds to the vehicle speed at which critical vibrations occur. However, vibration measurements can be taken at various speeds or in a range of speeds, as required for a vibration reduction by an effective balancing.
The invention combines the following features. A wheel unit is mounted on a balancing spindle that in turn is mounted on two oscillating bridges in a measuring unit. Two sensors detect the vibrations generated by the wheel unit to be balanced. An angular position sensor detects the angular position of the wheel unit where unbalances are present. The measured signals are processed in an evaluating unit. A wheel unit loading mechanism is mounted on the measuring unit and is so constructed that a controllable force or pressure is applied to the wheel unit. The size of the force is selected for optimally simulating the conditions between the tread contact area of a vehicle wheel tire and a road surface. The mounting of the loading mechanism is constructed to simulate the characteristics of the vehicle wheel suspension.
For achieving such simulation, the inertia mass of the loading mechanism corresponds to the unsprung mass of the wheel unit including the unsprung components such as the hub, the brake, and the wheel carrier. In order to simulate the actual rotational relationships in the balancing operation, the loading mechanism is provided with rotatable load elements in the form of rollers. According to the invention, the reduced mass m1dw of the rollers corresponds substantially to the reduced mass m2dR of the rotating parts of the vehicle wheel unit that are effective at the tread radius, thus: m1dw=m2dR as will be explained in more detail below.
In an advantageous embodiment, the contact force of the loading mechanism that simulates the actual vehicle operating conditions are controlled in open loop manner for simulation of various conditions that occur during vehicle travel.
The contact force can be reduced to zero by disengaging the loading device specifically the loading rollers from the tire tread, whereby the present apparatus can be used for conventional load-free dynamic wheel balancing.
In a particularly advantageous embodiment, the rollers of the loading mechanism are mounted in a roller shoe. The axes of rotation of the rollers run either parallel to or at an angle to the axis of rotation of the wheel unit. Further by rotating the roller shoe about an axis formed by the direction of loading, conditions can be simulated on the tire that are typical when the vehicle wheel deviates from straight-ahead travel. In another embodiment the roller shoe is tiltable for see-sawing about a journal axis extending horizontally through a vertical load application axis and in a plane passing at a right angle through the rotation axis of the wheel unit, whereby road curve conditions can also be simulated.
In another advantageous embodiment the rollers have roller sections of larger and smaller diameters. The sections of larger and smaller diameters on different neighboring rollers are offset or axially displaced so that a section of larger diameter of one roller extends radially into the space provided by a smaller diameter roller section between two larger roller sections on a neighboring roller. This intermeshing arrangement of the rollers makes the roller shoe compact and allows the rollers to be placed in relatively close proximity to each other, whereby the roller sections form a loading surface that contacts the tire tread at many closely spaced points within a single plane thereby optimally simulating a road surface. This arrangement has the further advantage that commercially available ball bearings or roller bearings can be used for forming the roller sections as rotatable roller rings. Particularly the roller sections of larger diameter can be formed as antifriction bearings, the outer races of which form the tire contacting load application surface.
Several rollers can be coupled together with regard to their rotational characteristics, whereby it becomes simpler to precisely determine the reduced rotational mass of the roller shoe.
After the balancing information has been obtained through an evaluating and display unit forming part of the balancing machine, it is preferred to rotate the wheel unit into a position in which a balancing weight can be conveniently attached to the wheel unit by the operator. Such a rotation of less than a full turn is possible by the wheel drive of the balancing machine. However, it is preferred to use a separate drive for this purpose. Such a separate drive includes a motor and at least one drive roller contacting the tire for turning the wheel unit to a specific angular position. A separate drive for this purpose allows for a simpler, more precise positioning of the wheel unit for the attachment of at least one balancing weight.